Described herein are radiation curable photochromic ink compositions ideally suited for use in ink jet ink printing devices. In embodiments, the photochromic ink includes a radiation curable ink vehicle, a photochromic material and an optional colorant. The ink vehicle may also contain one or more gelators and/or additional radiation curable components, along with an initiator for curing.
The volume of digital color printing is expected to experience significant growth in the coming years. The color images provided by ink jet printing inks are overwhelmingly preferred in panel studies over other digital imaging systems. There is also a strong case to be made that the total cost of ownership of an ink jet printer will ultimately be cheaper than similar volume electrophotography units.
Ink jetting devices are known in the art, and thus extensive description of such devices is not required herein. As described in U.S. Pat. No. 6,547,380, incorporated herein by reference, ink jet printing systems are generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field that adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium. There are three types of drop-on-demand ink jet systems. One type of drop-on-demand system is a piezoelectric device that has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. Another type of drop-on-demand system is known as acoustic ink printing. As is known, an acoustic beam exerts a radiation pressure against objects upon which it impinges. Thus, when an acoustic beam impinges on a free surface (i.e., liquid/air interface) of a pool of liquid from beneath, the radiation pressure which it exerts against the surface of the pool may reach a sufficiently high level to release individual droplets of liquid from the pool, despite the restraining force of surface tension. Focusing the beam on or near the surface of the pool intensifies the radiation pressure it exerts for a given amount of input power. Still another type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink vehicle (usually water) in the immediate vicinity to vaporize almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands.
In a typical design of a piezoelectric ink jet device utilizing phase change inks printing directly on a substrate or on an intermediate transfer member, such as the one described in U.S. Pat. No. 5,372,852, incorporated herein by reference, the image is applied by jetting appropriately colored inks during four to six rotations (incremental movements) of a substrate (an image receiving member or intermediate transfer member) with respect to the ink jetting head, that is, there is a small translation of the printhead with respect to the substrate in between each rotation. This approach simplifies the printhead design, and the small movements ensure good droplet registration. At the jet operating temperature, droplets of liquid ink are ejected from the printing device and, when the ink droplets contact the surface of the recording substrate, either directly or via an intermediate heated transfer belt or drum, they quickly solidify to form a predetermined pattern of solidified ink drops.
Conventional phase change hot melt inks typically used with ink jet printers of the aforementioned type frequently utilize a wax based ink vehicle, for example, a crystalline wax. Use of such crystalline waxes requires that the printhead be kept at least at 135° C. throughout printing with the device. The wax based inks are heated to such high temperatures to decrease their viscosity for proper jetting. Moreover, if the printhead is cooled and re-warmed, a lengthy purge cycle that consumes significant amounts of ink must be carried out. The brittle crystalline waxes also do not provide robust images and are easily scratched. This is because wax based inks generally crystallize at temperatures greater than room temperature and therefore, the wax based ink that has been transferred to the recording medium is essentially as hard as it will get. The high energy consumption, waste of expensive ink during purging, and fragile images all cause customer dissatisfaction, and in some markets prevent any sales penetration at all.
Recently, Xerox has discovered several radiation curable inks that may be jetted at much lower temperatures and that achieve robust images following curing. Reference is made to the following patent properties, each of which is incorporated herein by reference in its entirety. (1) Co-pending application Ser. No. 11/034,850 filed Jan. 14, 2005; (2) Co-pending application Ser. No. 11/034,856 filed Jan. 14, 2005; (3) Co-pending application Ser. No. 11/034,714 filed Jan. 14, 2005; (4) Co-pending application Ser. No. 11/034,866 filed Jan. 14, 2005; (5) Co-pending application Ser. No. 11/289,609 filed Nov. 30, 2005; and (6) Co-pending application Ser. No. 11/289,473 filed Nov. 30, 2005. U.S. Pat. Nos. 6,561,640 and 6,536,889, each incorporated herein by reference in its entirety, describe processes of forming ink jetted images using UV curable inks.
U.S. Pat. No. 5,892,116 (Weiss et al.) and PCT Patent Publication WO 97/24364 (Weiss et al.), the disclosures of each of which are totally incorporated herein by reference, disclose gelators that gel a variety of nonpolar and polar liquids. Moreover, gelation of various monomers with subsequent polymerization of the gelled monomers forms organic zeolites and membrane materials.
U.S. Pat. No. 5,593,486, incorporated herein by reference in its entirety, describes a hot melt ink composition comprising (a) an ink vehicle that is solid at about 25° C. and having a viscosity of from about 1 to about 20 centipoise at a temperature suitable for hot melt ink jet printing, the temperature being greater than about 45° C., (b) a photochromic material, (c) an optional colorant, and (d) an optional propellant
U.S. Pat. No. 6,329,453, incorporated herein by reference in its entirety, describes a photochromic phase change ink composition that contains (A) at least one selected polymeric and photochromic yellow phase change colorant and (B) a phase change ink carrier composition comprising (1) a resin selected from the group consisting of a urethane resin, a mixed urethane/urea resin and mixtures thereof; and (2) a mono-amide.
It is desirable in many commercial fields to provide validation or security markings to products or documents in order to make forgery or counterfeiting more difficult. U.S. Pat. No. 4,927,180, incorporated herein by reference in its entirety, describes a method of marking an article or substrate with a mark or device that is made visible or is enhanced on exposure to UV light, comprising printing onto the article or substrate a photochromic ink containing a photochromic compound that is reversibly converted to a colored form on exposure to UV light and reverts to a substantially colorless form in the absence of UV light, and applying a superficial protective layer to the mark or device to protect the photochromic compound against degradation by atmospheric moisture and oxygen.
While known compositions and processes are suitable for their intended purposes, a need remains for improvements in radiation curable inks, for example with respect to developing a radiation curable ink with photochromic capabilities.